Artificial turf—also known as artificial grass or artificial lawn game playing field—is usually provided as a carpet-like ground cover typically having more or less upright single filament or multifilament strands fastened to a substrate sheet and projecting upwardly from that sheet. The strands may be formed of thin, flat yarn of plastic material. The strands may be gathered into tufts that are fastened to the substrate sheet. The substrate sheet usually has a textile or textile-like structure and is usually made of plastic material as well.
Such ground covers usually cover a ground surface to form a game playing field and generally serve as a replacement for a natural grass ground cover, or other conventional playing field surface, which is less resistant to intensive use, requires more maintenance and generally forms a less uniform and more uneven surface.
A resilient underpad may be placed beneath the carpet and upon the firm ground support surface to provide a shock absorbing effect. Also known is the use of granular fillers on the substrate sheet and around the strands to promote an upright position of the strands. An example of such an artificial turf ground cover is described in international patent application WO98/56993.
The strands of such artificial turf type ground covers have to meet several requirements. One requirement is resilience, i.e. the strands have to spring back after having been pressed flat against the ground, for instance by the foot of a player, and the strands should be capable of springing back quickly and repeatedly. Also, especially for playing soccer, the strands should be sufficiently stiff to keep a ball lying thereon sufficiently spaced above the substrate layer. To meet these requirements, the strands are usually made of flexible, resilient materials that regains its original shape, also after important deformations, such as polypropylene, linear low density polyethylene or a block copolymer of polypropylene and polyethylene. Another requirement is that the strands have a good durability. In particular, the strands should not fibrillate too easily. Yet another requirement is that the friction coefficient between the strands and the human skin should not be too large to avoid skin-burns on human skin sliding over the grass. Especially, with respect to the latter property, improvement is still desired to make artificial turf attractive as a replacement for natural turf for games and other purposes, such as use a playground for children, which involve or may involve frequent sliding skin-grass contact such as soccer.
In European patent application 0 259 940, an artificial grass having a lower coefficient of friction than grass consisting of polypropylene strands is proposed. It is described to add polyethylene terephtalate to the polypropylene or to manufacture the strands of the grass of co-extruded multi-layered tape including an inner layer of polypropylene between outer layers of linear low-density polyethylene. Linear low-density polyethylene has a lower coefficient of friction than polypropylene and is known by itself as a material from which the strands may be manufactured. It is further described that, in addition, use can be made of polyethylene terephtalate that possesses a low coefficient of friction and has a (small) moisture absorbing capacity. However, such grass is relatively soft, which seems to have an adverse effect on load distribution such that relatively high normal pressures occur when a player falls on the grass. In combination with a still quite important grass-skin friction coefficient, this tends to cause skin abrasions upon sliding contact between the skin and the grass.
In international patent application WO 99/04074, it is proposed to combine polyamide and a polyolefin compound selected from the group consisting of polypropylene, linear low-density polyethylene and a block copolymer of polypropylene and polyethylene in a yarn for artificial turf. More in particular, co-extruded filaments are described, which contain a core layer or outer layers of polypropylene, linear low-density polyethylene or a block copolymer of polypropylene and polyethylene and outer layers or, respectively a core layer of polyamide. It is also described in this document to use yarns other than monofilaments, preferably, on the one hand, fibrillated yarn of the block copolymer of polypropylene and polyethylene, fibrillated yarn of high density polyethylene, fibrillated yarn of linear low density polyethylene or fibrillated yarn of polypropylene and, on the other hand, fibrillated polyamide yarn. However, also artificial turf made of such multifilament yarns does not reduce friction to the human skin to a sufficient degree for use as for example a soccer field. Furthermore, recovery of high-density polyethylene filaments after large deformations, such as pleating under the foot of a player, is poor.
In European patent application 0 417 832 it is described that after-fibrillation and the associated wearing-out of an artificial turf ground cover can be reduced by using fibres cut from extruded film, which have been stretched at a ratio of at most 1:2 to 1:3. However, this goes at the expense of fibre strength and stiffness that can be achieved at higher stretch ratio's, so that relatively much fibres material is required to achieve the desired overall firmness (i.e. ball carrying capacity) of the artificial turf surface.